How do I read it?

Summary: Find a blue block in the first row. There probably wont be any clouds
in the sky then.

Details: Read the image from left to right. Each column represents a
different hour. The first three colored blocks in the columns are the colors
from CMC's forecast maps for Kansas State University, for that hour. The two
numbers at the top of a column is the local time, in 24hr format, of that hour.
(Local time for Kansas State University is -5.0 hours from GMT.)

The line, labeled Transparency, is the transparency forecast. Here
'transparency' means just what astronomers mean by the word: the total
transparency of the atmosphere from ground to space. It's calculated from the
total amount of water vapor in the air. Dark blue means excellent transparency
befitting Arizona. Light blue is better than average and pale blue is worse than
average. White means that there is at least 20% cloud cover and transparency was
not calculated. Look at the cloud forecast for the same time to see how much
cloud there will be. The transparency forecast seems to be somewhat pessimistic.
CMC's text page explaining the this forecast is here.

The line, labeled Trans+Smog is an experimental version of the
transparency forecast with a smog included. Only clear sky clocks located near
major cities will show a difference between the transparency and
transpareny+smog forecats. It's more likely to see difference because the smog
forecast model is run only once a day, around 08:30T. That means that in the
afternoons, the transparency forecast will be much younger and therefore more
accurate than the transparency+smog forecast. That's because the
transparency forecast gets updated again around 12UT.

The line, labeled Seeing, is the astronomical seeing forecast. This
is an experimental forecast. Excellent seeing means at high magnification
you will see fine detail on planets and stars will show diffraction rings. In
bad seeing, planets might look like they are under a layer of rippling water and
show little detail at any magnification, but the view of galaxies is will
probably be undiminished. Bad seeing is caused by turbulence combined with
temperature differences in the atmosphere. This forecast attempts to predict
turbulence and temperature differences that affect seeing for all altitudes.

The excellent-to-bad seeing scale is calibrated for instruments in the 11 to
14 inch range. There are some more details in CMC's seeing
forecast page.

There are gaps in the line of seeing blocks because CMC's seeing model does
not consider daytime heating, so the forecast is only available for the night.
Seeing is forecast for 3 hour blocks, so triples of seeing blocks will show the
same color.

Note also that you may observe worse seeing though your telescope than what a
perfect seeing forecast would predict. That is because tube
currents and ground seeing mimic true atmospheric seeing. You may also
observe better seeing then predicted here when observing with an instrument
smaller than 11 inches.

The line labeled darkness is not a weather forecast. It shows when the
sky will be dark, assuming no light pollution and a clear sky. Black is a dark
sky. Deep blue shows interference from moonlight. Light blue is full moon.
Turquoise is twilight. Yellow is dusk and white is daylight. For those who
prefer numbers, the scale is also calibrated. Mouse over a darkness block for
details. The colors represent the limiting visual magnitude at the zenith. The
legend row at the bottom shows the magnitude that each color represents, from
mag 6, for a dark sky, to mag -4 for daylight. It is based on Ben Sugarman's Limiting
Magnitude calculations page. It takes into account the sun's and moon's
position, moonphase, solar cycle and contains a scattering model of the
atmosphere. It doesn't consider light pollution, dust, clouds, snow cover or the
observer's visual acuity. So your actual limiting magnitude will often be
different.